This invention relates to an expansion bolt of the type used to support the roof of a mine and, in particular, to a mine bolt assembly that has an anti-spinning feature that prevents the assembly from turning in a receiving hole as the bolt is being tightened.
One common form of mine bolt includes a tapered wedge that is threaded onto an elongated bolt. The wedge is arranged to coact with a hollow expansion shell that typically has a pair of opposed leaves. The wedge, in operation, is drawn by the bolt downwards into the shell to expand the leaves and thus drive them outwardly into anchoring contact against the wall of a receiving hole. The outer surfaces of the leaves are elongated and generally furnished with teeth that grip the receiving hole wall. Once anchored in place, the shell securely holds the wedge in place and allows the bolt to be further tightened to draw a support plate tightly against the roof of the mine shaft.
Typically, the leaves of the shell are joined by a channel or U-shaped sheet metal strap. The strap has a pair of parallel arms that are joined to the leaves and a perpendicular base member which passes over the wedge and the distal end of the bolt. A plastic sleeve holds the shell and the wedge together as a unit so that the entire assembly can be easily inserted into a receiving hole. The sleeve has a flange that is of greater diameter than the receiving hole and is stripped away from the assembly as it passes upwardly into the hole. The base of the strap engages the distal end of the bolt as it is threaded upwardly through the wedge whereupon the wedge is driven downwardly into the shell to spread the leaves outward into contact with the wall of a receiving hole. A breakaway plug may be placed in the base of the strap that separates from the strap at a desired bolt pressure thereby permitting the bolt to be further advanced. Although most mine bolts operate in the manner intended to anchor the bolt securely in a receiving hole, there are occasions when the leaves of the shell fail to extend properly and instead of the bolt advancing in the anchor, the entire assembly simply spins in the hole as the bolt is turned. It is a rather time consuming and expensive procedure to extract the "spinner" from the hole and replace it with a sound assembly. Spinners are believed to occur when the strap deforms in an unexpected manner which prevents the leaves from being extended.